The production of uniformly sheared streams by means of double gauzes in wind tunnels: a mathematical analysis

Abstract

The methods which have been used to design gauzes for the production of uniformly sheared streams in wind tunnels are analysed. The analysis of the results which have been obtained with single gauzes show that the poor theoretical evaluations of the shear rate that have been reported in the literature seem to be related to the different empirical expressions which have been assumed for the lift coefficient. This analysis indicates that the empirical relation established by Dryden and Schubauer (1947) gives satisfactory results, as long as mean mean angle of inclination of the gauze is taken into account in evaluating the resistance coefficient. The theoretical analysis of these flows shows that streams with “high” shear rates can only be obtained with single gauzes when “high” drag screens are used. This has the disadvantage of producing large scale turbulent motion downstream from the gauze. As an alternative to this single gauze approach, and to the semi-empirical technique used by Woo et al. (1981) of building a double gauze with non-uniform porosity, a theoretical analysis of the flow through double gauzes of uniform porosity leads to a useful method of designing gauzes for specific shear rates of high intensity. The method was used in the design of a double gauze that produced a stream with a non-dimensional shear rate of 1.73 (Woo et al. obtained 1.8), which is quite uniform and shows close agreement with the theoretical value. The results of this analysis indicate that the most appropriate method of producing high shear rates with low turbulence intensities is to use a set of several gauzes of low resistance. Modifications to the theoretical expressions obtained that take into account the presence of the additional gauzes might be a valuable improvement in the designing of gauzes for highly sheared streams with low turbulence intensities.